The growth and differentiation of epidermal cells in culture are being optimized by varying the medium, type and level of serum, antibiotic-antimycotic added, pH and temperature. Maintenance of epidermal morphology has been extended from about 7 to 14 days by lowering the temperature from 37 degrees to 31 degrees and the serum level from 11% to 2%. Growth of cell preparations containing mostly single cells is being compared to our usual preparations which contain only 10-20% single cells and many large clumps of cells. The goal of these studies is the cloning of mouse epidermal cells, which would facilitate transformation studies. Epidermal cultures have been used to study DNA repair after treatment with the skin carcinogens beta-propiolactone and N-methyl-N'-nitro-N-nitroso-guanidine. After low, relatively non-toxic levels of the carcinogens, repair was demonstrated with deoxyguanosine-3H, but not with thymidine-3H. Repair was seen with either precursor at high, toxic levels of carcinogen. The guanine-specific repair, which occurred under conditions where most cells survived, was demonstrated in mouse or rat fibroblasts and epidermal cells, but could not be found in human WI-38 cells. The relevance of guanine-specific DNA repair to carcinogenesis has not been determined since an in vitro epidermal transformation assay is lacking.